1. Field of the Invention
The present invention relates to an imaging unit, a color measuring device, an image forming apparatus, a color measuring system, and a color measuring method.
2. Description of the Related Art
An image forming apparatus, such as a color inkjet image forming apparatus or a color electrophotographic image forming apparatus, is becoming able to provide a higher image quality and is thus increasingly used for offset printing of advertising media, brochures, or the like, for which high-quality images are required although the number of copies to be printed is relatively small.
In the offset printing that requires a high image quality, in some cases, colors actually printed out by the image forming apparatus may differ from colors of a printing material desired by a customer.
In general, the customer checks the colors of the printing material on a display and then orders printing. However, the image forming apparatus has specific color reproduction characteristics according to each model, so that printed colors may differ from the colors checked on the display.
Therefore, a technology has been adopted to reproduce colors by using a color space such as the L*a*b* color space or the xyz color space that is independent of a device such as a display or an image forming apparatus.
The image forming apparatus controls the amount of color materials in order to output a designated color. For example, an inkjet image forming apparatus calculates an ink ejection amount or a print pattern to control the amount of ink ejected from an ink head, thereby controlling output colors. For another example, an electrophotographic image forming apparatus controls the amount of toner attached to a photoreceptor or controls the intensity of a laser beam, thereby controlling output colors.
However, the amount of color materials, such as the ink ejection amount of the inkjet image forming apparatus, varies depending on the state of a head nozzle, depending on a variation in the viscosity of ink, or depending on a variation in an ejection driving element (e.g., a piezoelectric element), resulting in a variation in the color reproducibility. Moreover, the ink ejection amount of the inkjet image forming apparatus may change over time in the same image forming apparatus or may vary between different image forming apparatuses. Therefore, the color reproducibility of an image varies over time or between image forming apparatuses.
Therefore, a conventional image forming apparatus performs a color adjustment process to reduce a variation in an output due to device-specific characteristics and improve the reproducibility of the output with respect to an input. For example, the color adjustment process is performed such that the image forming apparatus actually outputs a color patch image of a reference color (a reference color patch image) and a color measuring device measures the reference color patch image. The color measuring device generates a color conversion parameter based on a difference between a color measurement value of the reference color patch image and a color value of a corresponding reference color in the standard color space, and sets the color conversion parameter in the image forming apparatus. Thereafter, when outputting an image based on input image data, the image forming apparatus performs color conversion on the input image data based on the set color conversion parameter, and records or outputs an image based on the color-converted image data. Therefore, it is possible to prevent a variation in an output due to device-specific characteristics and output an image with higher color reproducibility.
In the conventional color adjustment process, a spectrophotometric color measuring device has widely been used as a color measuring device that measures colors of the reference color patch image. The spectrophotometric color measuring device can obtain the spectral reflectivity at each wavelength and thus can measure colors with high accuracy. However, the spectrophotometric color measuring device is expensive. Therefore, there is a need for a device that can perform color measurement with higher accuracy at lower costs.
Conventionally, a color measuring device has been proposed that includes a reference color measuring unit that measures a reference color patch in advance to obtain RGB data of a color reference value, a color image input unit that obtains RGB data by simultaneously or separately capturing a subject including the reference color patch and a color measurement target, an image extracting unit that extracts RGB data of the reference color patch and RGB data of the color measurement target from the RGB data obtained by the color image input unit, and a calculating unit that calculates a difference between the RGB data of the reference color patch obtained by the image extracting unit and the RGB data of the reference color patch obtained by the reference color measuring unit, and corrects at least the RGB data of the color measurement target based on the difference (see Japanese Patent No. 3129502). In the conventional technology, it is disclosed that a reference color patch to be compared with a subject being a color measurement target is placed near the subject, the subject and the reference color patch are simultaneously captured by a color video camera serving as the color image input unit, RGB data of the subject is corrected by using RGB data of the captured reference color patch, and the RGB data of the subject is converted to a color value in the standard color space.
However, in the technology described in Japanese Patent No. 3129502, it is difficult to maintain a constant positional relation among a subject, the reference color patch, and the color video camera. Therefore, imaging conditions may vary at every imaging and the imaging may not be performed stably.
Therefore, there is a need for an imaging unit, a color measuring device, an image forming apparatus, a color measuring system, and a color measuring method capable of stably capturing images.